If you are a .Mac subscriber, Apple appears to have activated the corresponding @me.com email addresses. If your email address was username@mac.com previously, you can now use username@me.com and emails should arrive in your .Mac mailbox. In the near future, Apple will be migrating all the email services over to Me.com, but users will be able to keep their @mac.com email addresses:
Yes. In addition to your mac.com email address, you will also get an address at me.com with the same username when MobileMe is available. For example, if your current email is emily_parker@mac.com, you will get emily_parker@me.com. You can send from whichever address you choose. The choice is yours. You will still receive email sent to your mac.com address so you won’t miss any emails.

Apple has also published Frequently Asked Questions about the .Mac to MobileMe transition.

The @me.com transition is part of Apple’s shift to MobileMe, a new web-based service that offers “desktop class” email, calendar and address book functionality. MobileMe is set to officially launch in early July.”